(i)Field of the Invention
This invention relates to a novel cochlea implant electrode and to a procedure for its manufacture, as well as to an article of manufacture for subsequent forming into such novel cochlea implant electrode and for a procedure for its manufacture.
(ii)Description of the Prior Art
The cochlea is a spiral bone in the temporal bone which contains the organ and nerves of hearing by which sound is perceived. At the present time, in various countries of the world, attempts are being made directly to stimulate the auditory nerves in the cochlea so that a person puffering from nerve deafness can "hear".
Electrodes of this kind have been described by Martin Sonn and Wolfgang Feist in an article entitled "A prototype flexible microelectrode array for implant-prosthesis applications" in Medical and Biological Engineering, Nov. 1974, pages 778-790. This prior art electrode is disposed to be inserted through an aperture in a turn of a patient's cochlea, whose internal ear is defective, in order to establish communication substantially in the area of the cochlea containing the acoustic nerves pertaining to that part of the audible spectrum, which is relevant to the intelligibility of speech.
Schindler et al, in "Multielectrode Intracochlear Implants" Arch Otolaryngol, Vol. 103, December 1977, discloses the use of spatial excitation of the cochlear nerve in cats.
Clark and Hallworth, in "A Multiple-electrode Array for Cochlear Implant", J. Laryngol, Otol 90/7, 1976 disclose a ribbon array including a plurality of elongated flat electrodes which are positioned along the length of the cochlea for stimulating the auditory nerve.
Similarly, bundles of thin wires have been employed by the Stanford Auditory prosthesis group by direct placement into the auditory nerve. Furthermore several other cochlea prostheses have been described in the prior art, including some where a connector is provided. See, for example, reports under NIH Contact NO. NO1-NS-7-2367, "Development of Multichannel Electrodes for an Auditory Prosthesis". Connectors for use and implantable medical electronic devices have also been described.
Several methods have been developed electrically to stimulate nerve fibers in the cochlea of a deaf person in a pattern roughly corresponding to longitudinal mechanical waves produced in the environment (which waves are heard as sound by persons with normal hearing) to thus produce the sensation of sound in the deaf person. The details and results of one such method are set forth in a document entitled "Cochlear Implants:. Progress and Perspectives" edited by William F. House and Karen I. Berliner, and which is Supplement 91-Volume 91, March-April 1982, No. 2, Part 3 of the Annals of Otology, Rhinology and Laryngology, publiphed by The Annals Publishing Company, 4949 Forest Park Blvd., St. Louis Mo., 63108, copyright 1982.
Generally such methods involve implanting an internal electromagnetic coil connected by leads to active and ground electrodes under the skin covering the person's skull and positioning the electrodes at predetermined locations in or adjacent the cochlea. An externally worn adjustable transducer is then used to produce electrical signals in response to longitudinal mechanical waves produced in the environment and received via a microphone on the device, and to couple those electrical signals to an external electromagnetic coil positioned on the outside of the skin opposite the coil under the skin. Electrical signals corresponding to those produced by the transducer are produced in the internal coil by magnetic coupling between the coils, and those signals stimulate the cochlea via the electrodes. While the signals do not produce the same sound sensations from the mechanical waves that the waves produce via the normal human ear, they do produce a range of sound sensations that, with training, can be used by the person to help identify the source of the waves and in many cases to help the person understand human speech.
Implanting the coil with its leads and electrodes under the skin covering the person's head generally comprises (1) exposing the outer surface of the person's temporal bone behind the ear, which is typically done by cutting an arcuate slit in the skin, muscle, and other tissue overlaying that bone, separating, and laying back the flap thus produced; (2) making an opening in the temporal bone through the mastoid and facial recess portions of the temporal bone between the outer surface of the temporal bone and the round window in the cochlea; (3) forming a recess for the coil via a surgical drilling device in the exposed squamous portion of the temporal bone; (4) locating and attaching the internal coil in the recess; (5) positioning the ground electrode in the eustachian tube or some other location such as the opening in the mastoid or under the temporalis muscle; (6) positioning the lead terminating at the active electrode through the surgically prepared opening with the electrode at a predetermined position relative to the cochlea (e.g., which position may be at the outside surface of the round window of the cochlea or inside the cochlea with the lead passing through the round window); (7) anchoring the leads in place through the use of an acceptable adhesive or by packing tissues from the head around them in the opening; and (8) then replacing the flap of skin, muscle, and other tissues by suturing it in place over the coil and opening, and allowing the slit that formed the flap to heal.
Other similar methods have involved the use of a plurality of leads which terminate at spaced active electrodes adapted to be inserted into the cochlea through the round window under the theory that selective activation of the electrodes may selectively stimulate different nerves in the cochlea and result in better sound discrimination by the person.
As noted above, in its usual form, a cochlear prosthesis consists of two parts implanted into the skull of the patient. The first part is an "electronics" package which is implanted in the mastoid bone behind the ear. The second part consists of an electrode assembly which is inserted into the cochlea in order to apply electrical stimulation to auditory nerve fibers. The electrode array or assembly must be electrically connected to the electronics package. In addition, an internally-worn transmitter/receiver device is used to transfer both information and power to the implanted unit, and may receive information telemetered back from the implant.
In order to stimulate the nerves it is necessary to insert at least one electrode in the cochlea and this insertion can be done through the round window or by drilling a hole into the cochlea. It is generally believed to be desirable to stimulate the nerves at various spaced positions along the length of the cochlea as the different frequencies perceived by a person with normal hearing are developed by stimulation of various nerves along the cochlea. The locations of these nerves have a relationship with the frequency perceived and, thus, it is essential to use an electrode array with electrodes at the required spaced positions and which, in itself, can be passed along the spiral of the cochlea.
As noted hereinabove, there have been previous forms of electrode arrays proposed, one of which was a stranded wire array in which electrodes were formed by the termination of the wires at various positions along the length of the array, the terminated wire being stripped and spirally wound around the other wires of the array. Such stranded electrodes provided the necessary electronic properties and, to a greater or lesser extent, the required mechanical properties. These electrodes were, however, found to be unsatisfactory as it was difficult to form them with a smooth outer surface and on insertion they tended to cause trauma. Sputtered arrays have also been proposed but it has been difficult and expensive to attempt to manufacture such arrays as they needed application of a very advanced form of technology.
Many problems have arisen in the implanting of such electrodes in the cochlea, and because of such problems numerous situations have been proposed. It was found, for example, that such an electrode had to be introduced into about nearly two turns of cochlea. It has appeared, however, that a satisfactory result was not attained, for while it was possible to bring the patient to such a condition that the patient could interpret electrical signals supplied through the electrode as being sound, such sound was not interpreted as being intelligible speech. This fact was said to be due to damaging the patient's acoustic nerves during the insertion of the electrode into the cochlea.
The invention in U.S. Pat. 4,261,372 patented Apr. 14, 1981 by C. C. Hansen et al was based on the acknowledgement that the entire area of cochlea which was relevant to speech could not be contacted collectively, because an electrode having a sufficient extension for this purpose would act unavoidably as a chisel, which during the introduction would gouge into cochlea and in this way damage the interior walls of the cochlea and possibly the acoustic nerves too.
That patentee alleged to provide such an electrode in which the draw-backs of the prior art electrode were obviated by providing an electrode in the form of a foil-like flexible, electrically insulating support member having nerve electrodes supported thereupon for implantation into the cochlea. The electrode established electrical communication to the acoustic nerves of the human ear, substantially in the area of the cochlea containing the auditory nerves pertaining to that part of the audible spectrum, which was relevant to the intelligibility of speech. The electrode had two prongs and a shank, each prong having a length which corresponds to the length of the turn in the cochlea into which it is to be inserted during implanation and hence the two prongs are of different lengths. Further, each prong terminated in a bevelled pilot guiding member which guided each prong through its respective turn in the cochlea, thereby to reduce damage to the auditory nerves. During implanation each prong passes through only a single turn of the cochlea and was prevented from gouging into the walls of the cochlea due to the provision of the bevelled guiding member. The guiding member could be provided as a drop-shaped member secured to the end of each prong or by forming an eyelet in the distal ends of the prongs.
These same patentees provided a further alleged improvement in the prior art cochlea electrode. In U.S. Pat. 4,284,085, patented Aug. 18, 1981 these patentees provided an electrode for implanation into the cochlea. Their invention provided an electrode, which had two conditions of curvature, the one of which was temporary and corresponded to the curvature in the middle of the particular turn of the cochlea and the other one of which was permanent and corresponded to the first position of the electrode in the cochlea in which position it obtains an optimum contact to the acoustic nerves. The electrode of that invention further contained elements for changing the condition of curvature from the temporary one to the permanent one when the electrode had been inserted into the cochlea. The permanent condition of curvature was constituted by the natural condition of curvature of the electrode, whereas the temporary condition of curvature was established by means of a detachable or loosenable connection, which, until it was detached or loosened, maintained the temporary condition of curvature.
Other patentees provided what were said to be further improvements on such electrodes. Thus, U.S. Pat. No. 3,752,929 disclosed the use of an electrode including a pair of elongated conductors for implanting in the cochlea. In addition, in U.S. Pat. No. 4,284,856, a multichannel auditory stimulation system was disclosed wherein selected cochlea excitation was achieved by using a multielectrode prosthetic device which was inserted in the scala tympani of the cochlea. Different tones could be perceived by the patient through selective excitation of the cochlea with the multi-electrode prosthetic device.
In respect of such cochea implants, it was noted that another problem was present, namely that of properly positioning the electrodes with respect to the cochlea and holding the electrodes in that desired position. Experience had shown that positioning of the active electrode often disrupted the delicate soft tissues of the cochlea (e.g., the basilar membrane or spiral ligament), and that a positioned electrode was sometimes inadvertently moved from the desired position during the operation. Also, it was found to be necessary sometimes to replace the internal coil during a revision surgery which was done by again making an arcuate incision and folding back a flap of skin, muscle and other tissues from over the internal coil, cutting the lead or leads connected to the internal coil, reconnecting a new internal coil, and saturing the flap in place over the new internal coil. Inadvertent movement of the electrodes occurred during such severing and reconnecting of the leads which could result in damage to the cochlea or an improper new position for the electrodes. Such movement occured because the anchor tissues packed around the leads did not attach firmly to the lead, and because of the slight force necessary to move the leads.
Solutions to such problems were said to be provided in U.S. Pat. No. 4,462,401 patented July 31, 1984 by P. A. Burgio, in U.S. Pat. No. 4,462,402 patented July 31, 1984 by P. A. Burgio et al, and in U.S. Pat. No. 4,487,210 patented Dec. 11, 1984 by W. L. Knudsen et al. In each of these patents, a method was provided for implanting electrodes in or around a human cochlea that involved inserting one end of an anchor in the temperal bone, and functionally engaging a lead to the electrode with the anchor. Numerous anchor embodiments were described together with tools for inserting some of the anchors.
Another major problem with a cochlea prosthesis was that entire replacement of the device was probably not feasible with present-day technology. The electrode assembly, once it has been implanted into the cochlea, probably could not be explanted without damage to the cochlea itself. The electrode assembly must thus be designed to have a long life (in the order of fifty years, or the expected life of the patient). However, it may be necessary or desirable to replace the electronics package, for example, due to a circuit failure, or to substitute a compatible assembly of more advanced design at some time in the future. Thus, permanent connections between the electronics package and the electrodes are not appropriate.
In a cochlea prosthesis, the connection problem is severe. Firstly, many connections were required between the electronics package and the electrodes in a cochlear prosthesis. Secondly, because of the confined space in the skull where the cochlear prosthesis was required to fit, the space constraints were severe. In addition, it was desirable for surgical convenience and safety to be able to make all connections between the electronics package and the electrodes simultaneously, with one procedure. Furthermore, the reconnection process must take place in an environment where fluid ingress cannot be prevented; since the electrode assembly remains permanently in the skull, each re-connection of the electrodes to a new electronics package must take place inside the patient's head.
U.S. Pat. No. 4,516,820 patented May 14, 1985 by J. Kuzma was said to solve some of the disadvantages of the above-noted prior art connector techniques. That patentee provided a cochlea prosthetic package having an electronics part and an electrode part. The two parts had ceramic plates with aligned, hermetically-sealed hollow-pin feedthroughs therein. The parts were connected by a silicone-elastic sheet having individual metal foil pieces extending therethrough, each piece of metal foil having its two ends bent over to lie flush against respective faces of the sheet. The metal foil ends contacted the feedthroughs to establish the electrical connections.
Another problem was to provide a sufficiently flexible electrode array. This problem was said to be solved by G. M. Clark et al in Canadian Pat. No. 1,115,352 issued Dec. 29, 1981. That patent provided an electrode array comprising a flexible, biologically-inert tube, a number of eletrodes conducting bands located at predetermined spaced distances along portion of the length of the tube, the electrodes lying, generally, within the diameter of the tube and a conducting wire associated with each electrode and passing to the interior of the tube through a slot, aperture or the like in the tube at a position beneath the associated electrode to which the wire is connected and along the length of the tube to one end thereof. If required, the tube could be filled completely or in part with a material with appropriate mechanical properties. Preferably each electrode was formed from a thin film of a biologically inert metal, e.g. platinum, and the tube could be of a biologically clean grade silicone rubber.